Jim,
Used to have the blinds closed all day every day for months. Then one day I thought of opening the blinds when the direct sunlight had passed so the sunlight wouldn't heat up the tanks but would still catch the natural light. Did not notice any additional algae problems.
Both the 42 gal and the 72 are under a year old. Am hoping the additional indirect light will promote coraline growth on the back walls, which, unfortunately, will also block the extra light going into the tanks
